A user equipment (UE) or a mobile node may be allowed to be registered to an IMS (IP Multimedia Subsystem) via different access technologies at the same time. For example WLAN (Wireless Local Area Network) access and UTRAN (UMTS Terrestrial Radio Access Network) access may be allowed in parallel. The parallel use of different access technologies may allow service continuity for example when a mobile user moves out of WLAN coverage and at the same time enters UTRAN coverage.
VCC (Voice Call Continuity) is a technology defined by 3GPP (3rd Generation Partnership Project) that provides a possibility to transfer a voice call ongoing in an IMS domain to a CS (circuit switched) domain and vice versa without disrupting the call. There are however unresolved issues relating to service continuity when transferring from one network access technology to another.
Let's assume the following scenario: First, a UE_A is registered to an IMS via WLAN (A_WLAN). Then, the user of the UE_A moves into an area where the WLAN coverage gets weaker but UTRAN is available. Therefore, the UE_A registers to the IMS via UTRAN GPRS (General Packet Radio Services) (A_GPRS) in parallel with the WLAN registration. In result, the UE_A has two parallel IMS registrations.
Now, the UE_A can start to transfer its ongoing SIP (Session Initiation Protocol) dialogs (that is, ongoing multimedia sessions, subscriptions e.g. to presence information, etc.) from A_WLAN to A_GPRS. This procedure may be referred to as PS-to-PS handover (packet switched-to-packet switched handover).
The UE_A may also have an ongoing multimedia session (for example, SIP INVITE initiated) that includes an audio part, as well as another RTP (Real Time Transport Protocol) transported media, such as a video share. In this case, the UE_A may desire that the audio part of this session is to be handled via a CS domain, whilst the video part should be handled via IMS (A_GPRS).
There are three major issues in this scenario:                1. IMS PS-to-PS handover,        2. Voice Call Continuity, and        3. splitting the voice from other media.        
It is not yet certain how these issues should be handled in order to ensure service continuity both for the voice and other media. It is likely that in current solutions one or the other of the voice and other media may be discontinued when performing the transfer. Thus further considerations are needed.